Up until the 1980's when railroads eliminated cabooses and changed the rules, when a train stopped on a main track it was the responsibility of the flagman to protect his train from being run into from behind by a following train. When the engineer was slowing to a stop he would sound his whistle with one long and three short toots. The flagman riding in the caboose upon hearing the whistle signal would drop off the back end of the train with his "flagging kit" (red flag, fusees, and torpedos) and walk back one mile (counting rails or telegraph poles to know how far he'd gone) to place two torpedos on the rail as a warning to a following train to slow down. He then walked half way back to his caboose and waited there for further instruction. As seen in this photo, this was done regardless of weather!

If a following train approached he'd signal it to stop short of hitting his own train from behind.

When his engineer was ready to resume running, the engineer would whistle 4 or 5 short toots. The flagman could then return to his caboose, as seen in the photo of him carrying his flagging kit.

Jack Delano had to traverse the US by train and take photos of the things he saw or did. That must have been some great fun but probably not very comfortable in terms of accomodations. Plus, he earned $2,300/yr. working for FSA. According to inflation calculators, that would be about $31,500 today.